Consistency index describes the state of cohesive soil relative to its liquid and plastic limits.

If $I_c = 1$, the natural water content is equal to the plastic limit ($w_n = w_P$).

If $I_c > 1$, the soil is in a semi-solid or solid state.

If $I_c < 0$, the soil is in a liquid state ($w_n > w_L$).

Provides a quick assessment of soil consistency in the field.

Useful for evaluating the engineering behavior of clays.

Limited to fine-grained soils (clays and silts).

Does not account for soil structure or sensitivity.

Foundation engineering design.

Predicting settlement characteristics of clayey deposits.

The consistency index is different from the Liquidity Index ($I_L$). $I_c + I_L = 1$.

Option B ($I_c=1$) corresponds to the plastic limit, not the liquid limit.